Armature Deform with sharp limits
I found a workaround: 
1) In edit mode, select the vertex group assigned to the bone and hit P to separate the selection from the main object
2) remove the vertices from the main object that belong to the bone (there's actually just one in this case)
3) In object mode select the main object and the separated one and hit ctrl-J in order to join them. Now if I select the vertex at the tip of the part to be moved, there is no influence shown to the main part vertices. Maybe there is a double vertex at that position, but that doesn't create any problem for me for now.